New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/35

Miocene stratigraphic relations and problems between the Abiquiu, Los Pinos, and Tesuque Formations near Ojo Caliente, northern Espanola Basin S. Judson May, 1984, pp. 129-135 in: Rio Grande Rift (Northern New Mexico), Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J.; [eds.], New Mexico Geological Society 35th Annual Fall Field Conference Guidebook, 379 p.

This is one of many related papers that were included in the 1984 NMGS Fall Field Conference Guidebook.

Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico.

Free Downloads

NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks.

Copyright Information

Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geological Society Guidebook, 35th Field Conference, Rio Grande Rift: Northern New Mexico. 1984 129 MIOCENESTRATIGRAPHICRELATIONSANDPROBLEMSBETWEENTHEABIQUIU, LOSPINOS,ANDTESUQUEFORMATIONSNEAROJOCALIENTE, NORTHERNESPANOLABASIN

S.JUDSONMAY Arco Exploration Company, Box 5540, Denver, Colorado 80217

INTRODUCTION near Ojo Caliente to over 300 m to the west and southwest in the The complex stratigraphic relations of the Abiquiu Formation, the Abiquiu area. The basal contact with the pre-Miocene rocks may drop mid-Miocene part of the Los Pinos Formation, and the Tesuque For- as much as 200 m because of pre-Abiquiu topography, allowing for the mation are locally well exposed in the northern Espanola Basin near greater thickness to the west. Ojo Caliente (Fig. 1). Exposures in this area show that much of the Los Pinos in the Ojo Caliente—Petaca area is equivalent in stratigraphic Contacts level and age to the Chama—El Rito Member of the Tesuque Formation Throughout most of the western Espanola Basin the Abiquiu uncon- (Fig. 2) and most or all of the Abiquiu Formation in the Espanola Basin. formably overlies pre-Miocene rocks. In the hills west of Ojo Caliente In the Ojo Caliente area the Abiquiu and Los Pinos represent a large the Abiquiu overlies a mature, early Miocene topography eroded on southwest-sloping fan complex of volcaniclastic conglomerate and Precambrian crystalline rock (see third-day road log, this guidebook). sandstone derived from rhyolitic and andesitic terranes to the north and This surface locally exhibits up to 300 m of relief. The Abiquiu probably northeast (Manley, 1981). The Tesuque Formation records a large ar- represents the initial major influx of elastic debris that filled the Espanola kosic accumulation of sand carried mainly west and south from Pre- Basin. cambrian rocks of the Sangre de Cristo uplift. In the Ojo Caliente area Smith (1938) and Smith and others (1961) recognized a gradational these two alluvial systems merged and intertongued in a complex man- upper contact of the Abiquiu with the overlying Santa Fe Formation. ner, producing the Chama—El Rito Member of the Tesuque Formation However, in the Ojo Caliente area, the contact they recognized is which contains interbedded volcaniclastic gravels and arkosic sand- considered here to be the upper contact of the Los Pinos Formation. stones of the two systems. Under the more restricted definition of the Abiquiu used here, the Abiquiu grades upward into the Los Pinos in the Ojo Caliente area. To ABIQUIUFORMATION the south and southwest it probably grades upward into the Chama—El The Abiquiu Formation (Abiquiu Tuff of Smith, 1938) consists of Rito Member of the Tesuque Formation of the Santa Fe Group (Fig. light-gray, tuffaceous sandstone and minor volcaniclastic conglomerate 2), although this relationship can be seen only west of El Rito Creek. exposed near the town of Abiquiu and distributed widely throughout Galusha and Blick (1971) also reported a slight angular unconformity the Abiquiu 30-min quadrangle. Only the uppermost part of the Abiquiu between the Abiquiu and Chama—El Rito near the "Ojo Caliente intru- is exposed at Ojo Caliente and is composed of andesite- and rhyolite- sive" southwest of the town, but the beds under the unconformity at pebble conglomerate containing sparse pebbles of Precambrian quartz- this location are the tuff-ring facies of the much younger Ojo Caliente ite, metarhyolite, gneiss, and schist. The Abiquiu of this report is tuff ring (May, 1980) and are located in the middle part of the ChamaEl restricted to the lower 0-80 m of the roughly 420 m mapped as Abiquiu Rito (Figs. 1, 2). by Smith (1938) in the Ojo Caliente area. The remaining upper 360 m In the Ojo Caliente area the only exposure of the upper contact is 3 of volcaniclastic conglomerate and tuffaceous sandstone is considered km west of town. Here the formation grades up into the Los Pinos to be the Los Pinos Formation (Cordito Member?) based on its strati- through a 40-m section. The very light-gray tuffaceous sandstones grad- graphic position and approximate lithologic similarity with the Cordito ually turn darker gray and the rhyolite- and andesite-pebble conglom- to the north. erates gradually increase to the point where they make up a major part of the section. The contact is chosen at approximately 15 m above the Lithology lowest volcanic-pebble conglomerate or where the conglomerates make Near Ojo Caliente, the Abiquiu contains mainly white, very light- up more than 25% of the section. Although a few of the conglomerates gray, light-brown, and minor red and green layers of tuffaceous sand- are locally erosional at their bases, the contact exhibits neither signif- stone. In the upper 20 m are a few layers of andesite- and rhyolite- icant erosion nor angularity between the underlying and overlying beds. pebble conglomerate, sparse fluvial crossbeds, laminar beds and chan- nel-fill structures. Sparse, discontinuous seams of mudstone occur lo- Age cally. In the lower 40 m, several dark lenses of conglomerate and breccia The Abiquiu ranges from 22 to 17-18 m.y. in age (early Miocene) contain subangular cobbles and fragments of locally derived Precam- near Ojo Caliente. Basalt flows of the Hinsdale Formation lie between brian metarhyolite. These lenses are lithologically similar to the Ritito the Abiquiu and the underlying Precambrian rocks along the north wall Conglomerate north of La Madera. The best local exposure is 3 km of Arroyo El Rito and have been K—Ar dated by Baldridge and others west—southwest of Ojo Caliente in a 70 m section along the north wall (1980) at 22.1 ± 0.6 m.y. These flows intertongue with thin discon- of Arroyo El Rito (Fig. 1). tinuous seams of light-gray tuffaceous sandstone, indicating Abiquiu The sandstone is variable in composition, consisting of medium to deposition during eruption of the flows. However, the lower part of the coarse-grained, subangular to subrounded quartz, K-feldspar and pla- Abiquiu in its type area is probably lower stratigraphically and older gioclase, with a few granular fragments of Tertiary sedimentary and than the Abiquiu in the Ojo Caliente hills, implying that it is also partly volcanic rocks and Precambrian metamorphic rock. Traces of opaque upper Oligocene. Vazzana (1980) inferred a 27-26 m.y. age for the minerals and altered biotite flakes also occur. Larger angular fragments base of the formation to the west. and granules of metarhyolite and pink feldspar occur in minor amounts Age of the upper contact is well constrained locally. Four km west throughout the sandstone, but generally decrease upward in size and of Ojo Caliente, a small olivine-nephelinite flow on the south slope of number. The fine-grained interstitial debris in most layers has a tuffa- Cerro Negro yielded a K—Ar date of 18.9 0.7 m.y. (Baldridge and ceous appearance. The Abiquiu appears to thicken greatly from a maximum of 90 m others, 1980); this flow occurs approximately 40 m below the top of roon and bluish-gray rhyolite and rhyolitic welded tuff, containing eu- the Abiquiu, implying an approximate 18 m.y. age for the top. Manley taxitic structure and phenocrysts of quartz. Clasts of light-gray quartz and Mehnert (1981) reported a 17.3 ± 0.8 m.y. K—Ar age on a dacitic latite(?) and andesite porphyry are also abundant; both contain phe- clast approximately 10 m below the top of the Abiquiu Formation 7 nocrysts of hornblende, light-gray feldspar, and sparse biotite and quartz. km northeast of Abiquiu. They also reported a 15.9 ± 0.9 m.y. K—Ar Minor granules and pebbles of soft, light-pink to light-gray andesite age on a dike that cuts the Abiquiu 6 km north of Sierra Negra, thereby and rhyolitic tuff breccia and a few zones with rounded boulders of bracketing the age of the upper contact between 17.3 and 15.9 m.y. porphyritic andesite(?) and rhyolite (0.3-1.0 m in diameter) imply one Since the dike is probably of the same age as the mid-Chama—EI Rito or more nearby volcanic sources for some of the debris. Minor angular volcanic field of May (this guidebook), the 17.3 m.y. date may ap- fragments of Precambrian metarhyolite, amphibolite, schist, and pink proximate the age of the upper contact most closely in this area. feldspar throughout the Los Pinos are slightly larger in average size than the more rounded volcanic clasts and are most common in the LOS PINOS FORMATION lower third of the section. The Los Pinos Formation, first named Los Pinos Gravel by Atwood Conglomerates are 0.1-3.0 m thick and generally have erosional and Mather (1932), is composed of conglomerate, sandstone and ash- bases exhibiting 0.1-0.3 m of relief. Fluvial crossbeds, slightly im- flow tuff. Interbedded basalt flows are part of the Hinsdale Formation bricated cobbles, and granular to pebbly volcaniclastic and arkosic (Lipman and Mehnert, 1975). On the east side of the Tusas Mountains, sandstone fill numerous paleochannels. Some conglomerates grade lat- the formation forms an eastward-thickening wedge of volcaniclastic erally into granular sandstone, and in many areas the sandstones and debris carried south by Miocene and Pliocene streams flowing out of conglomerates are finely interbedded. the late Tertiary San Juan volcanic field (Lipman and Mehnert, 1975). A tongue of Los Pinos conglomerate in the Chama—E1 Rito Member The Los Pinos (Cordito Member?) at Ojo Caliente was first mapped along the cliffs east of El Rito Creek (Fig. 1) is similar to the Los Pinos and correlated with the Conejos Formation by Atwood and Mather conglomerate at Ojo Caliente but contains more clasts of Precambrian (1932). These deposits were also assigned to the Abiquiu Tuff by Smith rock. In the southern part of the tongue, the conglomerate contains (1938). Three to five km south of Ojo Caliente, the Los Pinos grades about 75% rounded pebbles and cobbles of andesite, rhyolite, quartz laterally into the Chama—El Rito Member of the Tesuque Formation, latite, and dacite(?), and 25% subangular pebbles of quartzite, meta- which overlies the Abiquiu Formation in the Abiquiu area. rhyolite, and gneiss. As the tongue is traced northward along the cliffs, Bingler (1968) first mapped the Los Pinos at Ojo Caliente, but in- the pebbles of Precambrian rock gradually become larger (locally 0.3- cluded the upper 80 m of Abiquiu in his basal Los Pinos. In this report 1.0 m in diameter) and eventually comprise up to 90% of the debris the Los Pinos is restricted to those volcaniclastic conglomerates and in some layers near the latitude of Ojo Caliente. No northward increase sandstones (approximately 360 m thick) that overlie the light-gray tuf- in grain size or angularity of the volcanic clasts is noticeable. faceous sandstones of the Abiquiu Formation west of Ojo Caliente and Light-brown, pink, and pale-orange siltstones and sandstones that underlie a thin tongue of light-brown to pale-orange siltstones, sand- are interbedded with the Los Pinos conglomerates generally consist of stones, and gray volcanic conglomerates of the Chama—El Rito 1 km quartz, feldspar, and minor, small lithic grains in a slightly tuffaceous east of town. The Los Pinos is also recognized as a northward-thickening matrix. The layers are loosely cemented, evenly bedded, and are ap- tongue, 130-200 m thick, in the upper half of the Chama—El Rito along proximately similar in appearance to those of the Chama—El Rito al- the north-trending cliffs east of El Rito Creek (Figs. 1, 2). though the amount of tuffaceous debris decreases slightly to the south. Lithology Contacts In the Ojo Caliente area, the Los Pinos Formation is primarily pur- The Los Pinos unconformably overlies the Precambrian rocks near plish-gray volcanic-pebble conglomerate with numerous interbeds of Ojo Caliente along a surface of high relief, with stratigraphically higher slightly tuffaceous pink, light-brown, and pale-orange siltstone and levels of the Los Pinos overlying what were the higher elevations on sandstone that are similar to the Chama—El Rito. The basal 30 m of the hills before Neogene east-tilting of the region. In areas where the the formation contain numerous locally derived angular fragments of elevation on the surface of Precambrian rocks was low enough, the metarhyolite, granite, amphibolite gneiss, and muscovite schist where Abiquiu Formation underlies the Los Pinos along a gradational contact. it overlies hills of Precambrian rock. The conglomerates consist of granules, pebbles, and cobbles of ma- 132 MAY

The upper contact of the Los Pinos grades up into a thin tongue of m.y. old. upper Chama—El Rito 1 km east of Ojo Caliente and is located at the In southern Colorado, the Los Pinos underlies, and is interbedded uppermost major gravel layer in the pale-orange siltstones and sand- with, basalts of the Hinsdale Formation, from which Lipman and Meh- stones of the Chama—El Rito. Because most gravel layers are not con- nert (1975) reported radiometric ages of 25-5 m.y. Bingler (1968) also tinuous for more than 1 km, the uppermost major gravel occurs at reported a 25.9 m.y. date from a welded rhyolite tuff in the lower part slightly different levels in the section from one place to another, and of the Cordito Member west of Petaca. The Los Pinos is therefore upper the contact rises and falls as it is traced laterally. Oligocene to uppermost Miocene in the eastern Tusas Mountains and North of Ojo Caliente, the upper contact with the Chama—El Rito is is age-equivalent to the Abiquiu, Tesuque, and Chamita Formations to more complex. At Ojo Caliente, the Los Pinos conglomerates (360 m the south. thick) appear to be at approximately the same stratigraphic level as the middle to upper(?) Cordito Member north of La Madera, but are sep- HINSDALE FORMATION arated from it, at least in their upper part, by a laterally intervening Two exposures of basalt flows occur between the Precambrian and zone of the Chama—El Rito exposed on the north side of Canada de los Tertiary rocks in the hills west of Ojo Caliente; one at Arroyo El Rito, Comanches (Figs. 1, 2). Northward of Ojo Caliente, the volcaniclastic the second near Cerro Colorado. The basalts (formerly Jarita Basalt of conglomerates in the upper Los Pinos diminish abruptly and the upper Butler, 1946; Barker, 1958; May, 1980; Baldridge and others, 1980) contact drops in the section near the confluence of Canada de los are now included in the Hinsdale Formation in accordance with Lipman Comanches and Rio Ojo Caliente. Exposures along N.M. 96 for the and Mehnert (1975), who assigned all interlayered basalts of the Los first 3 km north of Canada de los Comanches contain more volcaniclastic Pinos Formation to the Hinsdale. The flows are at slightly different conglomerates than the overlying Chama—El Rito to the east, suggesting levels in the Tertiary section and appear to be small, isolated flows that that this gradational contact continues north at a lower stratigraphic were extruded into Miocene valleys from several local vents now buried level (parallel to the highway) for at least a few kilometers. However, by the Los Pinos and Abiquiu Formations. poor exposures between this area and La Madera make it difficult to demonstrate that the Los Pinos at Ojo Caliente is physically continuous The Arroyo El Rito exposure, the lower of the two, is 4 km S7OW with the Cordito to the north. The relationship is better exposed 8-10 of Ojo Caliente. There a series of four(?) superimposed flows overlies km west—northwest of Ojo Caliente in the hills east of El Rito, where the Precambrian rocks and underlies the Abiquiu Formation (Figs. 1, a complex intertonguing relationship occurs between the Los Pinos, 2). The greatest thickness of the flows is 30 m where they have filled Chama—E1 Rito and Abiquiu. a valley eroded on the Precambrian rocks. Elsewhere, the thickness is generally 15-20 m. Lateral extent of the flows is uncertain, but probably The Los Pinos exposed at Ojo Caliente grades south and upward into is greater than that seen in the exposures. To the north, stratigraphically the Chama—El Rito Member of the Tesuque Formation. Three to five higher parts of the Abiquiu directly overlie the Precambrian. Some km south of Ojo Caliente the number of conglomerate layers diminishes flows are separated from each other by thin, discontinuous zones of and the more arkosic pale-orange siltstones and sandstones of the Chama- light-gray, tuffaceous sandstone (0-1.2 m thick) similar to the Abiquiu El Rito dominate the section from here south into the Espanola Basin. Formation. A few volcaniclastic conglomerates extend many kilometers to the south, Chemical analyses and norm calculations for the flows indicate a and tongues of pink and brown sandstone, up to 25 m thick (typical of quartz-tholeiite composition with 53% SiO2 and 3.8% normative quartz the Chama—El Rito), extend north beyond La Madera, thereby making (Baldridge and others, 1980). Most major-element oxides are within this laterally gradational contact difficult to locate precisely. Of all 1% of those reported by Barker (1958) for his Jarita Basalt at Tusas contacts in the Tertiary section, the contact between the Los Pinos and Creek in the Las Tablas quadrangle. Similar SiO2 values at 51 and 52% Chama—El Rito is the most subject to reinterpretation or relocation by were reported by Lipman and Mehnert (1975) and Lipman and others other workers, depending on where the most significant lithologic change (1978) for basalts in the Hinsdale Formation. is thought to occur. Several flows of the basalt also occur on the southeast flank of Cerro The tongue of Los Pinos in the cliffs east of El Rito Creek grades Colorado between the Precambrian rock and the overlying Los Pinos south into the upper half of the Chama—El Rito and is thought to be a Formation. The basalt is 0-35 m thick and is exposed intermittently west—southwest extension of the thick gravels at Ojo Caliente. The for a distance of 3 km (Fig. 2). The dark-gray flows are deeply weathered lower contact of the tongue, although gradational, occurs as a relatively olive green and purple in most outcrops and locally exhibit spheroidal sharp upward increase in the abundance of conglomerate near the base weathering. Petrographically, the flows are approximately similar to of the cliffs. The contact appears to drop in the section northward near the flows along Arroyo El Rito, but are approximately 65 m higher in the latitude of Ojo Caliente. the section. They could be slightly younger than the flows at Arroyo The upper contact of the tongue is exposed very poorly a short El Rito or could have erupted at the same time, but at an elevation 65 distance east of the cliffs, but with difficulty can be traced to the north m higher on the Precambrian rocks, thereby not being buried until the along a large easterly dip-slope in the upper Chama—El Rito. At the basal Los Pinos was deposited. latitude of Ojo Caliente this contact rises stratigraphically at the expense of the overlying Chama—El Rito and is only about 35 m below the base of the Ojo Caliente Sandstone, similarly to the situation east of Ojo TESUQUE FORMATION (CHAMA—EL RITO MEMBER) Caliente. The Chama—El Rito Member of the Tesuque Formation was named by Galusha and Blick (1971) for buff to pink sandstone, siltstone, and Age interbedded purplish-gray, lenticular layers of volcaniclastic conglom- Age of the Cordito(?) at Ojo Caliente ranges from 18 to approximately erate exposed in the badlands north of Rio Chama, 5 km east of its 12 m.y. (mid-Miocene). The base of the member is approximately 40 confluence with El Rito Creek. These beds are confined to the western m stratigraphically above an olivine-nephelinite flow dated at 18.9 ± and northern parts of the Espanola Basin and grade north into the 0.7 m.y. by Baldridge and others (1980), and is probably 17-18 m.y. Cordito(?) Member of the Los Pinos near Ojo Caliente. old. The approximate 12 m.y. age for the upper contact is not well con- Lithology strained near Ojo Caliente. Southeast of town the upper contact is about The Chama—El Rito is primarily a sequence of arkosic siltstone, 110 m higher stratigraphically than the 15.3 m.y. old Ojo Caliente tuff sandstone, and lesser volcaniclastic conglomerate. Minor beds of white ring (May, this guidebook) and very approximately 250 m lower than and pale yellowish-green airfall tuff and tuffaceous sandstone, thin beds basalt flows in the lower Chamita Formation near Chili (24 km south) of reddish-brown silty clay, and very sparse layers of white limestone dated at 9.6 ± 0.2 m.y. by Baldridge and others (1980). The middle occur mainly in the upper 200 m in the badlands north of Rio Chama, of the Los Pinos tongue exposed along the cliffs east of El Rito Creek implying lower paleoslopes to the southwest. is about 170 m above a 15.3 m.y. old volcanic vent (date by Ekas and others, this guidebook) exposed along the creek, and is probably 1314 MIOCENESTRATIGRAPHICRELATIONS 133

South of Ojo Caliente the Chama—El Rito is 460-550(?) m thick and determined by: typically occurs in widely spaced, fault-controlled exposures in the (1) pebble imbrication in paleochannels, structurally higher areas. It also forms a tongue (35-60 m thick) between (2) dip direction of foreset crossbeds, and the Los Pinos Formation and Ojo Caliente Sandstone 1 km east of Ojo (3) correlation of channels between outcrops. Caliente. Two km north of Ojo Caliente the member thickens abruptly Channels west of El Rito Creek trend almost due south, whereas those to more than 150 m along a laterally gradational contact with the to the east near lower Rio Ojo Caliente trend more to the southwest. underlying Los Pinos conglomerates and is well exposed in the badlands This convergence may indicate that the axis of the basin trended north— on the north side of Canada de los Comanches. south during the mid-Miocene and was near the longitude of lower El The bulk of the Chama—El Rito is a sequence of silty to medium- Rito Creek. Possible minor east-tilting of the west side of the basin grained, moderately to well-sorted, arkosic to lithic sandstone beds may have caused this southerly diversion of the southwest-flowing colored light-orange brown, orange-pink, and salmon. Layers are me- streams near Ojo Caliente. dium-bedded to massive, with even, parallel bedding. Mineral com- Several thin, widely spaced layers of white, very light-gray and pale position is variable, but typical amounts are 50-70% quartz, 20-30% yellowish-green airfall tuff and tuffaceous sandstone occur in the upper plagioclase and K-feldspar, 5-15% volcanic and lesser plutonic rock 90 m of the Chama—El Rito and lower 60 m of the overlying Ojo fragments, 1-3% opaque minerals, and 1-2% other miscellaneous grains Caliente Sandstone. The tuffs first appear about 6-8 km south of Ojo including secondary(?) clay. In some beds, the sand-sized fraction con- Caliente and increase slightly in abundance southward toward Rio Chama tains glassy and tuffaceous grains (20-35%). The sandstones are typ- and beyond. ically friable to crumbly, slightly tuffaceous, and weakly cemented with In the badlands west of lowermost Rio Ojo Caliente, several white minor clay, hematite, and calcite. Interbeds of loosely cemented, red- and very light-gray limestones (0-1.5 m thick) occur 45-125 m below dish-brown, sandy siltstone (0.1-3.0 m thick) occur in the south near the top of the Chama—El Rito, implying low paleoslopes and proximity Rio Chama along with very minor beds of dune-like sandstone. to the basin center. Most layers are traceable for 90-1000 m, and, in Volcaniclastic conglomerates occur throughout the Chama—El Rito some areas, several discontinuous beds occur at various levels in the section, but a few systematic variations are noticeable. Conglomerates section. Although no fossils were identified, the beds appear to be comprise up to 60% of the Los Pinos at Ojo Caliente, but become intensely bioturbated by worm(?) tubes and contain mudcracks(?). In thinner, fewer, and slightly finer-grained as they are traced south and thin section, the tubes are filled with mosaic, sparry calcite and fibrous southwest into the Chama—El Rito. They make up less than 10% of the chert. The matrix is calcareous mud (micrite). Scattered throughout the section in the south near Rio Chama. In most areas, the conglomerate mud are sparse rounded quartz grains (eolian?) and possible fragments are sparse or absent in the upper 25-35 m. Five to six km southwest of fossil algae and CaCO3. The sparse limestones may be deposits of of Ojo Caliente, an ill-defined gravelly zone (90-120 m thick) in the small playas or shallow depressions of standing saline water on the Chama—El Rito is about 70 m stratigraphically above the Ojo Caliente interfluvial plains and are absent to the north near Ojo Caliente. tuff ring of May (this guidebook) and is probably equivalent to the tongue of Los Pinos along the El Rito cliffs. Contacts The conglomerates contain subangular to rounded granules, pebbles, The Chama—El Rito grades down into the Abiquiu Formation through- and small cobbles of rhyolite ash-flow tuff, light-gray porphyritic an- out most of the area; grades north into the Cordito(?) Member of the desite, dacite, quartz latite(?), and Precambrian blue and gray quartzite, Los Pinos Formation; and grades up into the overlying Ojo Caliente brown metarhyolite, granite, quartz, and green to black gneiss and Sandstone as a series of tongues of orange-brown siltstone of the Chama— amphibolite. Rock types are given above in very approximate order of El Rito and light-brown eolian sand of the Ojo Caliente Sandstone. decreasing abundance, although relative amounts are variable through- out the region and no systematic study of the variations was attempted. Age Clasts of the Tertiary volcanics are by far the most abundant except In the Ojo Caliente—Rio Chama area, the Chama—El Rito is laterally near hills of Precambrian rock and are generally more rounded than equivalent to the Cordito(?) Member of the Los Pinos. Here the Cor- those of the Precambrian. The gravels are poorly sorted and generally dito(?) has an age range of 18-12 my. and establishes an approximately have a light-gray or light-brown sandy matrix rich in quartz and lesser similar age range for the Chama—El Rito in this area. A 15.3 m.y. old feldspar, rock fragments, and minor tuffaceous debris. A detailed pet- volcanic horizon occurs in lower middle part of the Chama—E1 Rito rographic study of the conglomerates, as also suggested by Galusha between Ojo Caliente and lower El Rito Creek (Ekas and others, and and Blick (1971), will provide needed additional insight on the source May, this guidebook). of these deposits (Ekas and others, and Dethier and Martin, this guide- book). CORRELATIONOF UNITS Geometry of the conglomerates is variable. Most layers grade lat- Correlation of the Miocene units is complicated by contacts that erally and upward into beds of nonvolcanic sandstone. Many have sharp intertongue, grade laterally and are time-transgressive as a result of erosional bases and, in some areas, a subtle upward-fining grain size. evolving uplifts, and by basin geometry and source areas. Figure 2 Some form continuous sheet-like layers (3-6 m thick) that are traceable summarizes the stratigraphic relationships between La Madera and the for up to 1 km (especially in the south near Rio Chama) and that have Rio Chama. only 0.1-1.0 m of erosional relief at their bases. Within these larger The lower 0-80 m of section west of Ojo Caliente consist of white tabular layers are numerous small stringers (0.1-1.0 m thick) of slightly tuffaceous sandstone (here considered to be upper Abiquiu) which is imbricated pebbles and cobbles that extend laterally 5-30 m, but do probably similar in stratigraphic level and age to part of the upper not scour the underlying beds. Abiquiu in its type area. The lower few meters of metarhyolite con- Numerous paleochannels (generally 2-8 m wide) exhibit 0.1-0.6 m glomerate and breccia in the section at Ojo Caliente may be physically of erosional relief at their bases, although a few cut down as much as continuous with, but younger than, the 90 m thick basal granitic gravel 1.0-1.2 m. These channels generally form at the base of the larger of the Abiquiu in the type area. This unit is also tentatively correlated tubular layers. A few also occur as small, lenticular channels (3-15 m with the Ritito Conglomerate to the north, which Vazzana (1980) con- across) that are isolated in the sandstone. The bottom of a few of the sidered to be basal Abiquiu. The Abiquiu in the type area probably more deeply entrenched channels is lined with lag zones of imbricated thinned to the north, as it was deposited on the hilly topography of the cobbles. In some channels, trough-shaped crossbeds in the sandstone former Laramide Brazos uplift in the Ojo Caliente area during the early dip 5-20° into the channel axes, approximately conforming to the floor Miocene. of the channels. Foreset beds are sparse. Few, if any, slump structures The Abiquiu as defined here is age-equivalent with, and appears to occur on the channel walls, and flute casts and ripple marks in the finer- grade northeast into, the lower Cordito in the El Rito—Ortega Mountain grained layers are very sparse. area, based on limited reconnaissance work (May, 1980). However, Paleotrend measurements on eight channels show a high directional variability and an average S2OW direction (Fig. 1). Directions are 134 MAY careful study of Abiquiu — Los Pinos — Chama—El Rito relationships is Los Pinos conglomerates at Ojo Caliente are similar in appearance to still needed in this critical area of intertonguing. The Upper Member the Cordito and contain clasts of rhyolitic ash-flow tuff, more petro- of the Abiquiu Formation (of Vazzana, 1980; Vazzana and Ingersoll, graphic study of the clasts is needed to positively identify them as the 1981) contains clasts of ash-flow tuff similar to those of the Cordito, Cordito Member. implying that the conglomerates of the upper Abiquiu are southwestern Near Ojo Caliente, the Los Pinos streams maintained southwesterly tongues of the Cordito (Manley, 1981). However, 3 km west of Ojo courses as they flowed around the southern end of the former Laramide Caliente the Abiquiu clearly grades upward into the Los Pinos along a Brazos uplift. This is indicated by several lines of evidence: contact that is about 18 m.y old. South of Ojo Caliente the Abiquiu (1) thickness, abundance, and grain size of conglomerate layers in underlies, and in most places grades up into, the Chama—El Rito. the Los Pinos (and laterally equivalent Chama—El Rito) decrease Reconnaissance work north of Ojo Caliente suggests that the 360 m to the south and southwest; of volcaniclastic conglomerate at Ojo Caliente I mapped as upper Abi- (2) imbricated pebbles in the Los Pinos fluvial channels have a slight quiu Tuff by Smith (1938), but considered here to be Cordito(?)] is tendency to dip northeast in many exposures (no systematic study approximately equivalent in stratigraphic level and probably in age to was made, however); the middle part of the Cordito Member as defined by Barker (1958) in (3) on the southeastern flank of Cerro Colorado, erosional debris of the Las Tablas quadrangle and Butler (1946, 1971) in the eastern Tusas Hinsdale basalt flows in the basal Los Pinos occurs southwest Mountains. These conglomerates are thought to be coarser-grained, of Hinsdale exposures; stratigraphically higher and younger than the Abiquiu in its type area (4) abundance, grain size, and angularity of quartzite, metarhyolite, near Abiquiu, and grade laterally to the south into the Chama—El Rito and gneissic rubble in the Los Pinos tongue at El Rito cliffs Member of the Tesuque Formation. increase northward, culminating southwest of Cerro Colorado and La Madera Mountain, which formed major topographic highs SOURCEAREASANDDEPOSITIONALENVIRONMENTS of Precambrian rock during Los Pinos deposition. During the Miocene, the Ojo Caliente area was near the southern By 18 m.y. ago, shortly after eruption of the olivine-nephelinite flow end of a large alluvial apron derived from volcanic terranes in the San at Cerro Negro, the Abiquiu alluvial system was buried by the Los Juan Mountains and the Questa—Taos area (Lipman and Mehnert, 1975; Pinos (Cordito Member?) in the Ojo Caliente area. Shortly thereafter, Manley, 1981). The Brazos uplift, which was still a low northwest- light-brown and pale-orange beds of arkosic sandstone occurred in Los trending highland of Precambrian rock, was gradually being buried by Pinos gravels of the Ojo Caliente area and were abundant in the Chama— Abiquiu and Los Pinos alluvium at this time. In broadest terms, this El Rito to the south. This change at about 18 my. might be a slightly volcanic system merged and intertongued with a dominantly arkosic delayed effect of uplift of the Sangre de Cristo Range 1-2 my. earlier, alluvial system (Tesuque Formation) brought west and southwest from which according to Manley (1981) was already the site of a volcanic the Sangre de Cristo Mountains. edifice in the Questa area supplying debris to the lower Los Pinos The Abiquiu Formation formed a broad alluvial deposit derived from Formation. Local advance of the Los Pinos gravels over the finer- streams originating in a rhyolitic to andesitic volcanic region to the grained Abiquiu system at Ojo Caliente occurred simultaneously with north and northeast (Vazzana and Ingersoll, 1981). The tuffaceous al- introduction of the arkosic Chama—El Rito above the tuffaceous Abiquiu luvium buried a widespread erosion surface formed on hills of Precam- farther south at this time. This implies uplift of an already present brian rock in the western Brazos uplift and on lower Tertiary and volcanic terrane in the Sangre de Cristos to the northeast near Questa Mesozoic sedimentary rocks to the southwest where relief was more and simultaneous uplift of a granitic and sedimentary terrane farther subdued. The debris entered a mildly sagging basin (early Espanola south along the range. Basin) whose boundaries were wider than its present fault boundaries, The Chama—El Rito south of Ojo Caliente accumulated in the more as indicated by the presence of Abiquiu on the eastern flanks of the central part of the Espanola Basin as it was simultaneously receiving Nacimiento uplift and at Cerro Pedernal, both of which are west of the arkosic sand and andesitic to rhyolitic gravel from the two source areas. fault margin of the basin. Paleochannel measurements of the lower The south- and southwest-flowing streams were probably shallow and Abiquiu indicate southwesterly stream flow, and measurements in the wide, forming a braided distributary(?) system, as indicated by the upper Abiquiu indicate southeasterly flow away from the San Juan lateral extent of many gravel layers and the scarcity of cut-and-fill volcanic field along the western side of the Brazos highland, perhaps structures more than 0.6 m deep. Sparse lenticular sandstone layers at as a result of early east-tilting of the western Espanola Basin (Vazzana, various levels in the conglomerates may represent small sandbars be- 1980; Vazzana and Ingersoll, 1981). Clasts of ash-flow tuff are similar tween the migrating channels. Ripple marks and fluvial dunes are sparse to those of the Los Pinos, possibly implying a common source area and upper flow-regime bedforms absent. A slight southwesterly decrease and that the Abiquiu may have simply been a more distal, finer-grained in average grain size and in the number of cut-and-fill channels also part of the early Los Pinos depositional complex (Manley, 1981). indicate a slight southwesterly decrease in stream turbulence and com- The Los Pinos formed a widespread alluvial deposit derived from petence. areas consisting of rhyolitic and andesitic ash-flow tuffs and flows. The numerous fine-grained sandstones of the Chama—El Rito prob- Several distant and local volcanic sources are indicated. In the south- ably were deposited on broad, flat areas between the distributary chan- eastern San Juan Mountains, the Los Pinos forms a debris apron around nels (interfluvial plains), but depositional mechanisms are uncertain. late-stage volcanic cones and domes along the northern rim of the The sandstone layers may have been deposited by short-lived surges of Platoro caldera (Lipman and Steven, 1970) and "accumulated as great sand-laden water overflowing the channels, forming sheet floods across coalescing alluvial fans along the east front of the San Juan Mountains; the interfluvial plains. In support of this interpretation, the sandstones accumulation was a response to eastward tilting of the Oligocene vol- are finer-grained and more evenly bedded than the conglomerate layers canic sequence concurrent with structural development of the San Luis and have a laterally gradational relationship with some of the conglom- Valley graben" (Lipman, 1975). According to Manley (1981), P. W. erates. An eolian origin for some of the fine-grained sandstones and Lipman identified clasts of rhyolitic ash-flow tuff in the Los Pinos that siltstones is supported by good sorting, a lack of upward-fining texture, are similar to volcanic units in the Questa area to the northeast. In the and a massive appearance similar to loess. Therefore, paleoslopes south eastern Tusas Mountains, Manley also noted clasts derived from local of Ojo Caliente gradually flattened into the Espanola Basin during the outcrops of ash-flow tuff that erupted from the Questa area. A few large mid-Miocene, as indicated by lacustrine limestones in the south, con- cobbles and boulders of light-gray, rhyolitic ash-flow tuff and por- vergent paleostream trends, and finer grain size. phyritic andesite in the Ojo Caliente area also indicate local erosional sources for a small amount of the debris. ACKNOWLEDGMENTS Manley (1981) noted a greater amount of porphyritic, intermediate- The above text is modified from part of my Ph.D. dissertation done composition volcanic clasts in the Chama—El Rito than in the Cordito, at the University of New Mexico under direction of L. A. Woodward which contains abundant clasts of rhyolitic ash-flow tuff, and suggested different sources for the volcanic debris in the two units. Although the MIOCENESTRATIGRAPHICRELATIONS 135 and supported by USGS grants 14-08-0001-G-255 and 14-08-000I-G- tains: Geological Society of America, Memoir 144, pp. 119-154. 348, New Mexico Bureau of Mines and Mineral Resources grant 900- __, and Steven, T. A., 1970, Reconnaissance geology and economic sig- 60-84, and New Mexico Energy Research and Development grant I 17- nificance of the Platoro caldera, southeastern San Juan Mountains, Colo- 69-B. The paper was critically reviewed and improved by K. Manley rado: U.S. Geological Survey, Professional Paper 700-C, pp. C19-C29. and R. V. Ingersoll. _, Doe, B. R., Hedge, C. E., and Steven, T. A.. 1978, Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence: Geological Society of America, Bulletin, v. 89, pp. 59-82. Manley, K., 1981, Redefinition and description of the Los Pinos Formation of REFERENCES north-central New Mexico: Geological Society of America, Bulletin, v. 92, Atwood, W. W., and Mater, K. F., 1932, Physiography and Quaternary geology pp.984-989. of the San Juan Mountains, Colorado: U.S. Geological Survey, Professional May, S. J., 1980, Neogene geology of the Ojo Caliente-Rio Chama area, Paper 166, 176 pp. Espanola Basin, New Mexico: Ph.D. thesis, University of New Mexico, Baldridge, W. S., Damon, P. E. Shafiqullah, M., and Bridwell, R. J., 1980, Albuquerque, 204 pp. Evolution of the Rio Grande rift, New Mexico; new potassium-argon ages: Muehlberger, W. R., 1979, The Embudo fault between Pilar and Arroyo Hondo, Earth and Planetary Science Letters, v. 51, pp. 309-321. New Mexico: an active intracontinental transform fault: New Mexico Geo- Barker, F., 1958, Precambrian and Tertiary geology of Las Tablas quadrangle, logical Society, Guidebook 30, pp. 77-82. New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bul- Smith, C. T., Budding, A. J., and Pitrat, C. W., 1961, Geology of the south- letin 45, 104 pp. eastern part of the Chama Basin: New Mexico Bureau of Mines and Mineral Bingler, E. C., 1968, Geology and mineral resources of Rio Arriba County, Resources, Bulletin 75, 57 pp. New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bul- Smith, H. T. U., 1938, Tertiary geology of the Abiquiu quadrangle, Rio Arriba letin 91, 158 pp. County, New Mexico: Journal of Geology, v. 46, pp. 933-965. Butler, A. P., 1946, Tertiary and Quatemary geology of the Tusas-Tres Piedras Steinpress, M. G., 1981, Neogene stratigraphy and structure of the Dixon area, area, New Mexico (abstract): Geological Society of America, Bulletin, v. Espanola Basin, north-central New Mexico: summary: Geological Society 57, no. 12, p. 1183. of America, Bulletin, pt. 1, v. 92, pp. 1023-1026. _, 1971, Tertiary volcanic stratigraphy of the eastern Tusas Mountains, Vazzana, M. E., 1980, Stratigraphy, sedimentary petrology and basin evolution southwest of the San Luis Valley, Colorado-New Mexico: New Mexico of the Abiquiu Formation, north-central New Mexico: M.S. thesis, Uni- Geological Society, Guidebook 30, pp. 289-300. versity of New Mexico, Albuquerque, 115 pp. Galusha, T., and Blick, J. C., 1971, Stratigraphy of the Santa Fe Group, New _, and Ingersoll, R. V., 1981, Stratigraphy, sedimentology, petrology and Mexico: American Museum of Natural History, Bulletin, v. 144, art. 1, basin evolution of the Abiquiu Formation (Oligo-Miocene), north-central 127 pp. New Mexico: summary: Geological Society of America, Bulletin, pt. I, v. Lipman, P. W., 1975, Evolution of the Platoro caldera complex and related 92, pp. 990-993. volcanic rocks, southeastern San Juan Mountains, Colorado: U.S. Geolog- ical Survey, Professional Paper 852, 128 pp. ______, and Mehnert, H. H., 1975, Late Cenozoic basaltic volcanism and development of the Rio Grande depression in the southern Rocky Moun-